1. Field of the Invention
This invention relates to antenna arrangements and particularly to radar antennas mounted within radomes.
2. Description of Related Art
Target seeking missiles incorporating such arrangements are generally required to operate at high speed, when aerodynamic considerations demand a high fineness ratio radome profile, i.e., an acutely pointed profile as shown in FIG. 1 of the accompanying drawings.
At relatively small angles off the antenna boresight, which is assumed aligned with the missile longitudinal axis, target signals passing through the radome will have a high incidence angle at the inside surface of the opposite wall and a significant proportion of the incident energy will be reflected. The reflected signal may intercept the antenna which will then be receiving signals both direct and after reflection. A wide beam-width antenna, which is desirable in such systems, will detect both signals and the interference between them will result in a degradation of the radiation patterns.
The antenna assembly may comprise pairs of antenna elements for tracking a target in azimuth and elevation. As a result of the above degradation, the tracking response of a pair of antennas will suffer perturbation to the desired monotonic (within the operating field of view), and approximately linear, difference characteristic. Minor perturbations are manifested as ripples on the tracking response which cause a localised tracking error; major perturbations result in a reversal of the slope of the tracking curve (difference characteristic) and lead to ambiguous target bearing information.
The above problem is one of several which are exacerbated by the use of acutely pointed radomes but which may occur to a lesser extent with more traditional, continuous profile radomes. Amongst these problems are the following:
1. Direct reflection from the internal wall of the radome--as discussed above. PA0 2. Multiple reflections between the antenna and the radome wall. PA0 3. Surface wave effects along the radome wall with signals close to grazing incidence. PA0 4. Scattering and coupling between antenna elements.